(1) Field of the Invention
The present invention relates to method for sensing a light emissive element in an active matrix display pixel cell. It also relates to an active matrix display, comprising a plurality of pixel cells each having a current driven light emissive element such as an organic or polymer light emitting diode and a data line connectable to a drive element and to an electrode of the emissive element.
(2) Description of Related Art
Defects or structural inhomogeneities, for example particles from the substrates or from the processing of the device and pinholes and hillocks in the layers, are a severe problem for the lifetime of all OLED displays (including polymer and small molecules, segmented, passive matrix and active matrix displays).
Initial screening and burn-in procedures can be applied to reduce defects appearing during the manufacturing process, but such defects can also be activated during the lifetime of the display.
A selection criteria for identification of any defective pixels in a matrix display during the initial screening and during operation has previously been proposed in WO 01/22504. According to this technique, the stability of the OLED can be checked by applying a reverse voltage over the OLED and detecting the resulting leakage current variation over time. Such a leakage current is small in the ideal device, but will be significantly larger if a defect is present. Therefore, defective pixels can be identified. On the contrary, in forward mode when the diode is ON, the current flowing through the diode is large, and any current contribution from a defect is hidden. This is illustrated in FIG. 1.
The same effect can be utilized for using the pixel as a sensor. When subject to external influence, such as light, temperature, color, radiation or physical contact, the leakage current of the OLED will be altered. This alteration can be detected in the same way as mentioned above with regards to defects in the OLED.
Techniques for correcting pixel defects have also been proposed for passive and active matrix displays. Strong voltage pulses are applied to an OLED in reverse mode. This high field can induce a high current to either heal or isolate a defect in a pixel.
In the case of active matrix, a simple circuit with two transistors (the addressing and the driving transistor) is considered. The pixel circuit is voltage controlled through the data line by the column driver. In normal addressing, after selection of the pixel the voltage is written to the store point, and this controls the current flowing through the driving transistor to the OLED from the power line. Therefore the OLED emits light according to the voltage supplied to the store point.
In this case known techniques for correcting defects consists of applying on the power line a voltage which is negative with respect to the OLED cathode. Thus a negative voltage is provided across the driving transistor and the OLED. When the OLED is reverse biased in such a way, the current flowing through the driving transistor is usually much smaller than when the OLED is forward biased and therefore the driving transistor is only slightly open. In order to have maximum voltage drop across the OLED the driving transistor should operate in linear mode. In this way the source-drain voltage is minimized. However, since the voltage of the OLED anode is not directly controlled and the transistor is very wide (=capable of large current even at low voltage) operation of the transistor in linear mode is very difficult to realize.